I have a torque tube 35' - 36' banjo rear in my AV-8. For each revolution of the rear tire the speedometer gear turns 3/4 of a turn. I've installed mini gages in the factory instrument panel as I am still using the stock fuel tank and trying to keep the dash looking simple and close to original. I have small matching a steering column mounted tach. I found a small matching mechanical speedo that is available from the Drag Specialties motorcycle catalog that I can column mount next to the tach. The catalog says that the speedo is available in 1:1 (originally drove off Harley transmission) or 2:1 ratio (originally drove off Harley front wheel) Don't care if it doesn't read exactly accurate as I will clock it to determine 25, 35, 65 mph etc.. Which ratio should i use to get the closest? Any ideas how I can make this work?
Hummmm ..... 1000 revs per mile = 1000 revs per 5280 feet which I can convert to cir***ference of the tire based on the diameter. May be able to calc something with this.
The 2:1 wheel-driven Harley speedos turn about 1560 turns per mile, based on a 80" cir***ference motorcycle front tire that's used on just about everything. Typical Japanese bikes are anywhere from 1720 to 2280 turns per mile, although there's virtually no aftermarket mechanical speedos available for those.
Thanks Steve, this also helps. Not sure how my speedo gear only turning 3/4:1 compared to the wheels will factor in quite yet. See you had some Jags. A friend of mine had an E-type coupe with small block chevy, was pretty quick and handled well.
So if a car speedo normally turns 1000/mile and the bike speedo turns 1560 turns per mile, maybe the speedo gear at 3/4:1 will slow it doen enough to turn close to accurate?
Has to be a speedometer expert somewhere on the HAMB that knows all the relationships of this stuff. You guys know of anybody?
So an 80" cir***ference is about at 25.5" diamater wheel. My rear tires are 30.5" diameter or 95.8" cir***ference. Which means to travel the same distance, the wheel has to turn less times.
If it was mine....I'd get a car speedo, and build the car. Then when it's finally on the road, I'd see how accurate the odometer is, and if it needed a change, I'd build a ratio adapter box to fix it. You could probably use the same approach with the motorbike speedo, too.
Yeh agree, the car is very close. Down to a few simple wires and the fuel line, then ready to roll. Like the bike speedo cuz its small and real good match.
Car speedos are geared to a 'standard' 1000 cable revs = 1 mile. That doesn't mean you're stuck with that; you could install whatever speedo gears in the trans you need to make the speedo read right if the right ratio is available. The Harley 1:1 ratio will be closer to what you need at 780 wheel turns per mile, with your tires you've got around 660 turns. That's only about 15% off in terms of tire revs, you might be able to get a gear that will get you close. With your current 3/4 turn ratio, that's about a 40% difference so you need a driven gear with 40% more teeth on it. You can also get 'corrector' gearboxes that go between the speedo and the trans to fix almost any mismatch. You'll probably need custom cables, but that shouldn't be a big problem. Yeah, I had a Jag infatuation for a few years, but the Lucas electrics finally beat me down... And I'm a retired electrician...
Got ya, your explanation is way helpfull. Only problem is the spedo gear drives off the torque tube drive shaft and is not easily changed like a chevy trans speedo gear can be. Cable still screws on the same way. I see there a few dirrerent cable threads. So the 'corrector' would be the way to go. So I guess I'll start out with the 1:1 and see from there. Thanks!
http://www.gaugeguys.com/speedcorrect.htm Towards the bottom of the page is the ratio changers. You can find these things on Ebay much cheaper, but this shows what changes the ratio for you.
Also there are at least 4 driven gears available for the Ford tube, sorted of course by rear ratio and tire size...easy external bolt on there. Several are finally available in repro. You can probably ballpark it from the Ford catalog chart by figuring out which 1940's tire is closest to whatever you have in cir***ference, see how it all works, THEN decide whether you need any final tuning. The Ford stuff is for the normal 1,000 turns per mile. Here's a good test page: http://www.speedometershop.com/rep-pag.htm That outfit also makes custom drive cables and ***embles ratio adapters to suit. You can also hook up whatever decent speedo you have to your cable without fitting the gauge to the dash just to see a reading with OEM Speedo, then hook it to the aftermarket part.
Thanks Bruce, will look at this too. The different driven gears is what I first had in mind but no source for.
Just found out I can get the mini-speedometer I want to use in any of three available ratios: 1:1 2:1 or 2240:60 So which would be best to start with? Was thinking 1:1 based on Steve's previuos comment.
As said several times cars use the 1000 caliberation ! that is what you want,then a ratio correction box if needed.
Use the 1:1 speedometer, get it hooked up and run it on a measured mile and let me know how many 1/10's your turning in a mile. Then I can build you a ratio adapter to calibrate it. Posted using the Full Custom H.A.M.B. App!
source for some ratios of driven gear: Here and adjacent pages at Bratton's. http://www.brattons.com/prodtype.as...rPageHistory=&strKeywords=&strSearchCriteria= '30-31 type uses modern cable...note that he lists only by ratio, since it is a resto site ***uming stock wheel diameter, you need chart from parts book to correlate fully different combos of tire and ratio. The 3.54 basic gear is rare, so repro is a goody, 3.78 and 4.ii are common as dirt and you can find them on HAMB almost certainly.
That would be the simple way to be able to run that speedometer but in the end the cost may be more than buying an actual automotive speedometer close to the same size. The calibration boxes are available through most speedometer shops and semi truck shops and 4x4 shops where they are used all the time to correct speedometers when they change to different size tires on the trucks. You will have to have a cable made up that has the fitting for the bike speedo on one end and the adapter on the other though. That may end up being the hard part of the whole deal.
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